Method and apparatus for switching radio links in mobile communication

ABSTRACT

A radio link switching scheme in which a communication between the base station and the mobile station is carried out through the radio link given in terms of frames while providing a switching timing information formed by a layer 1 bit data in each frame of the radio link such as a unique word, a flag, a housekeeping bit, and a frame number. Then, the switching timing information in at least one of the frames of the radio link at the base station is changed to indicate a switching of the radio link whenever a need for switching the radio link between the base station and the mobile station arises, and transmitted to the mobile station. At the base station, the radio link is switched at a switching timing which is a prescribed period of time after each frame in which the switching timing information is changed, while at the mobile station the radio link is switched at a switching timing which is a predetermined period of time after each frame in which the changed switching timing information is detected.

TECHNICAL FIELD

The present invention relates to a radio link switching scheme suitablefor an intra-base station radio link switching.

BACKGROUND ART

In recent years, the mobile communication for carrying outcommunications by utilizing radio links between base stations and mobilestations has come to be widely used. The service area of such a mobilecommunication is formed by a multiplicity of cells (or zones), where abase station is provided in each cell. Thus, usually, the mobile stationwhich moves within a certain cell is going to carry out thecommunication through a prescribed radio link with the base stationprovided in that certain cell. Here, the radio link used is notnecessarily fixed to a particular one, and will be switched as the needarises.

There are two types of the radio link switching required in the mobilecommunication, including the inter-base station radio link switching andan intra-base station radio link switching. The inter-base station radiolink switching is executed in a case of changing the base station incommunication with the mobile station, as the mobile station moves froma cell of the currently communicating base station to a neighboring cellof another base station. On the other hand, the intra-base station radiolink switching is executed in a case the degradation of the radio linkquality is caused by the interference from the other base station ormobile station. A conventional scheme for this intra-base station radiolink switching will now be described.

In the digital mobile telephone system currently in practical use, theintra-base station radio link switching within one and the same basestation is handled by the identical scheme as in the inter-base stationradio link switching, which proceeds according to the sequence chartshown in FIG. 1 as follows.

Namely, while the base station and the mobile station are incommunication by using the transceiver circuit of the base station, whenthis transceiver circuit detects the degradation of the quality of theradio link in use, this fact is notified to a control circuit of thebase station in a form of a link switching request (step S101). Inresponse, the control circuit makes the selection of unused wire link,frequency, and time-slot (step S103), and notifies the selected wirelink to an exchanger station. Then, the exchange station makes thesignal transmission with respect to the notified wire link in additionto the wire link currently in use (step S105) while the control circuitnotifies the selected frequency and the time-slot to the transceivercircuit connected with the selected wire link.

The transceiver circuit which received this notice then generates thenew radio link synchronization signal according to the notifiedfrequency and the time-slot, and transmits the generated new radio linksynchronization signal to the mobile station (step S107). In addition,the control circuit transmits the new radio link designation signal tothe mobile station via the old radio link so as to notify the frequencyand the time-slot of the new radio link (step S109).

Then, the control circuit of the mobile station notifies the notifiednew frequency and time-slot to the transceiver circuit of the mobilestation such that this transceiver circuit switches the frequency andtime-slot to the notified new frequency and time-slot (step S111). Then,this transceiver circuit receives the new radio link synchronizationsignal transmitted from the transceiver circuit of the base station tocarry out the synchronization establishing processing for the new radiolink (step S113), so as to establish the new radio link synchronizationon the mobile station side and the base station side (steps S115 andS117).

When this synchronization is established, the base station stops thesignal transmission to the old wire link, and releases the old wire linkand radio link (step S119). Then, the communication through the newlyset up radio link is started.

Now, in contrast to the intra-base station radio link switching in theTDMA-FDMA system in which it suffices to change only the radio sectionparameters given by the frequency and the time-slot, the above describedconventional intra-base station radio link switching in the mobilecommunication system uses two transceiver circuits on the mobile stationside and the base station side, so that it also becomes necessary tochange the wire link connected with the transceiver circuit of the basestation side, and for this reason the switching processing isconsiderably more complicated.

In order to eliminate the necessity for changing the wire link, it isnecessary to switch the radio section parameters within one and the sametransceiver circuit. Here, in a case of switching the radio sectionparameters within one and the same transceiver circuit, if the switchingtimings are different at the base station side and the mobile stationside, there is a high probability for the breaking of the communicationover a considerable period of time to occur between the completion ofthe switching on one side and the completion of the switching on theother side. For this reason, it is preferable to carry out the radiolink switching at the same timing on both the base station side and themobile station side. However, in the above described conventionalscheme, there is no means for notifying the switching timing among thebase station and the mobile station, so that it has been difficult tocarry out the radio link switching at the same timing on both the basestation side and the mobile station side.

Moreover, even when the use of two transceiver circuits as in the abovedescribed conventional scheme is accepted, there still remains theproblem of the breaking of the communication between the base stationand the mobile station over a period of time required since theswitching of the frequency and time-slot at the transceiver circuit ofthe mobile station to the new values selected at the base station untilthe establishment of the synchronization for the new radio link.

DISCLOSURE OF THE INVENTION

It is therefore an object of the present invention to provide a radiolink switching scheme capable of carrying out the radio link switchingat the same timing on both the base station and the mobile station,eliminating the need for a processing with respect to the wire link, andpreventing the breaking of the communication at a time of the radio linkswitching.

It is another object of the present invention to provide a radio linkswitching scheme capable of reducing a probability of the failure of theradio link switching, carrying out the radio link switching atappropriate timings, and causing no trouble in the frame order controlat the mobile station side.

According to one aspect of the present invention there is provided amethod for switching a radio link in a mobile communication between abase station and a mobile station, comprising the steps of: carrying outa communication between the base station and the mobile station throughthe radio link given in terms of frames while providing a switchingtiming information formed by a layer 1 bit data in each frame of theradio link; changing the switching timing information in at least one ofthe frames of the radio link at the base station to indicate a switchingof the radio link whenever a need for switching the radio link betweenthe base station and the mobile station arises; transmitting the changedswitching timing information from the base station to the mobilestation; switching the radio link at the base station to a new radiolink at a switching timing which is a prescribed period of time aftereach frame in which the switching timing information is changed at thechanging step; detecting the changed switching timing informationtransmitted from the base station at the mobile station; and switchingthe radio link at the mobile station to the new radio link at aswitching timing which is a predetermined period of time after eachframe in which the changed switching timing information is detected atthe detecting step.

According to another aspect of the present invention there is provided amobile communication system including a base station and a mobilestation, comprising: means for carrying out a communication between thebase station and the mobile station through the radio link given interms of frames while providing a switching timing information formed bya layer 1 bit data in each frame of the radio link; means for changingthe switching timing information in at least one of the frames of theradio link at the base station to indicate a switching of the radio linkwhenever a need for switching the radio link between the base stationand the mobile station arises; means for transmitting the changedswitching timing information from the base station to the mobilestation; means for switching the radio link at the base station to a newradio link at a switching timing which is a prescribed period of timeafter each frame in which the switching timing information is changed bythe changing means; means for detecting the changed switching timinginformation transmitted from the base station at the mobile station; andmeans for switching the radio link at the mobile station to the newradio link at a switching timing which is a predetermined period of timeafter each frame in which the changed switching timing information isdetected by the detecting means.

Other features and advantages of the present invention will becomeapparent from the following description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sequence chart for a conventional intra-base station radiolink switching scheme.

FIG. 2A is a schematic block diagram of a base station of a mobilecommunication system according to the present invention.

FIG. 2B is a schematic block diagram of a transceiver unit in the basestation of FIG. 2A.

FIG. 3A is a schematic block diagram of a mobile station of a mobilecommunication system according to the present invention.

FIG. 3B is a schematic block diagram of a transceiver unit in the mobilestation of FIG. 3A.

FIG. 4 is a sequence chart for the first embodiment of a radio linkswitching scheme according to the present invention.

FIG. 5 is a signal form diagram for a new spread code designation signalused in the scheme of FIG. 4.

FIG. 6 is a diagram of transmission signals in the scheme of FIG. 4 forexplaining the first case of using a unique word as the switching timinginformation.

FIG. 7 is a diagram of transmission signals in the scheme of FIG. 4 forexplaining a modification of the first case of using a unique word asthe switching timing information.

FIG. 8 is a diagram of a transmission signal in the scheme of FIG. 4 forexplaining the second case of using a flag in each frame as theswitching timing information.

FIG. 9 is a diagram of a transmission signal in the scheme of FIG. 4 forexplaining one modification of the second case of using a flag in eachframe as the switching timing information.

FIG. 10 is a diagram of a transmission signal in the scheme of FIG. 4for explaining another modification of the second case of using a flagin each frame as the switching timing information.

FIG. 11 is a signal form diagram of a transmission signal in the secondcase of using a flag in each frame as the switching timing information.

FIG. 12 is a diagram of a transmission signal in the scheme of FIG. 4for explaining the third case of using a flag in a housekeeping bit asthe switching timing information.

FIG. 13 is a diagram of a transmission signal in the scheme of FIG. 4for explaining one modification of the third case of using a flag in ahousekeeping bit as the switching timing information.

FIG. 14 is a diagram of a transmission signal in the scheme of FIG. 4for explaining another modification of the third case of using a flag ina housekeeping bit as the switching timing information.

FIG. 15 is a signal form diagram of a transmission signal in the thirdcase of using a flag in a housekeeping bit as the switching timinginformation.

FIG. 16 is a diagram of a transmission signal in the scheme of FIG. 4for explaining a frame number.

FIG. 17 is a diagram of a transmission signal in the scheme of FIG. 4for explaining the fourth case of using a frame number as the switchingtiming information.

FIG. 18 is a diagram of a transmission signal in the scheme of FIG. 4for explaining one modification of the fourth case of using a framenumber as the switching timing information.

FIG. 19 is a signal form diagram of a transmission signal in the fourthcase of using a frame number as the switching timing information.

FIG. 20 is a sequence chart for the second embodiment of a radio linkswitching scheme according to the present invention.

FIG. 21 is a diagrammatic illustration of a table used in the scheme ofFIG. 20.

FIG. 22 is a diagram of a transmission signal in the scheme of FIG. 20for explaining one possible relationship between the switching timingnotice frames and the switching timing.

FIG. 23 is a diagram of a transmission signal in the scheme of FIG. 20for explaining another possible relationship between the switchingtiming notice frames and the switching timing.

FIG. 24 is a diagram of a transmission signal in the scheme of FIG. 20for explaining still another possible relationship between the switchingtiming notice frames and the switching timing.

FIG. 25 is a sequence chart for a modification of the second embodimentof a radio link switching scheme according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Now, the embodiments of the radio link switching scheme in the mobilecommunication according to the present invention will be described.

It is to be noted first that, in the mobile communication, a pluralityof mobile stations make communications with one base station, so thatthe multiple access scheme is adopted. Here, the multiple access schemeincludes the frequency division multiple access (FDMA) scheme, the timedivision multiple access (TDMA) scheme, and a code division multipleaccess (CDMA) scheme, and each of these may be realized separately, orin combination with the others. In the present invention, it is possibleto use any of these schemes as long as the radio link has a framestructure capable of containing the layer 1 bit data representing theswitching timing information as will be described below. The onlydifference that arises due to the choice of the multiple access schemewill be the parameter for specifying the radio link, which is given bythe frequency in the FDMA, the time-slot in the TDMA, and the spreadcode in the CDMA. In the following, the embodiments of the presentinvention will be described for an exemplary case of using the CDMAscheme for the sake of explanation, although the present invention isequally applicable to the FDMA scheme or the TDMA scheme.

In this case, the base station suitable for the radio link switchingscheme of the present invention has a configuration as shown in FIG. 2A,while the mobile station suitable for the radio link switching scheme ofthe present invention has a configuration as shown in FIG. 3A. In FIGS.2A and 3A, the solid lines indicate communication lines while the dashedlines indicate control lines.

In the configuration of FIG. 2A, the base station comprises: an antenna11 for transmitting and receiving radio signals to and from the mobilestation; a common amplifier 13 for amplifying the transmitting signalsand the receiving signals; a plurality (N pieces) of transceiver units15 (15₁, 15₂,-, 15_(N)) for encoding and modulating the transmittingsignals, as well as decoding and demodulating the receiving signals: anda base station control unit 17 for carrying out a control of theoperation in the base station and a generation of a control signal forthe mobile station.

Here, each transceiver unit 15 corresponds to one radio link. As shownin FIG. 2B, each transceiver unit 15 includes a switching timing set upunit 15a, a switching timing information changing unit 15b, a spreadcode switching unit 15c, and a control unit 15d for controlling theoperation in the transceiver unit 15. In addition, each transceiver unit15 is connected with a wire link for transmitting speech or data fromthe user which is connected to an exchange station.

On the other hand, in the configuration of FIG. 3A, the mobile stationcomprises: an antenna 21 for transmitting and receiving radio signals toand from the base station; an amplifier 23 for amplifying thetransmitting signals and the receiving signals; a transceiver unit 25for encoding and modulating the transmitting signals, as well asdecoding and demodulating the receiving signals; a mobile stationcontrol unit 27 for carrying out a control of the operation in themobile station and a generation of a control signal for the basestation; an operation unit 29 formed by a man-machine interface such asdial buttons; and an input/output circuit 31 for carrying out input andoutput processing for the speech or the user data.

Here, as shown in FIG. 3B, the transceiver unit 25 includes a switchingtiming information change detection unit 25a, a switching timing set upunit 25b, a spread code switching unit 25c, and a control unit 25d forcontrolling the operation in the transceiver unit 25.

Next, the first embodiment of the radio link switching scheme accordingto the present invention will be described with reference to thesequence chart of FIG. 4.

Namely, in this first embodiment, the radio link switching scheme iscarried out according to the sequence chart of FIG. 4 as follows.

Initially, it is assumed that the transceiver unit 15 of the basestation and the transceiver unit 25 of the mobile station is incommunication by setting up a certain radio link.

Then, when the transceiver unit 15 of the base station detects thequality degradation of that certain radio link (step S1), thetransceiver unit 15 notifies this quality degradation to the controlunit 17 of the base station (step S3).

In response, the control unit 17 selects the unused spread codes of theswitching target for each of the reverse link (mobile station→basestation) and the forward link (base station→mobile station) (step S5),and transmits the new spread code designation signal containing theselected spread codes to the mobile station by using the radio linkbefore the switching (step S7). Here, the new spread code designationsignal has an exemplary form as shown in FIG. 5 which comprises a signaltype 31 indicating that this signal is the new spread code designationsignal, a reverse spread code number 32 indicating the selected spreadcode for the reverse link, and a forward spread code number 33indicating the selected spread code for the foward link.

Then, the control unit 27 of the mobile station which received this newspread code designation signal stores the reverse and forward spreadcode numbers 32 and 33, and returns the new spread code designationconfirmation signal as an acknowledge to the base station (step S9).

Next, the control unit 17 of the base station which received the newspread code designation confirmation signal from the control unit 27 ofthe mobile station then notifies the spread codes for the reverse andforward links to be used in the new radio link to the transceiver unit15 (step S11). The transceiver unit 15 of the base station whichreceived this notice then stores the notified spread codes for thereverse and forward links, and returns the new spread code confirmationto the control unit 17 (step S13).

Next, in the transceiver unit 15 of the base station, an appropriateswitching timing is set up at the switching timing set up unit 15a (stepS15), and the switching timing information contained in a frame in aprescribed relation with respect to the switching timing set up at theswitching timing set up unit 15a is changed in a manner described indetail below by the switching timing information changing unit 15b andthe frame containing the changed switching timing information istransmitted to the transceiver unit 25 of the mobile station (step S17).

Then, in the transceiver unit 25 of the mobile station, whether theswitching timing information contained in the received frame is changedor not is checked by the switching timing information change detectionunit 25a, and when the switching timing information change is detected(step S21), the switching timing is set up at the switching timing setup unit 25b in a prescribed relation with respect to the framecontaining the changed switching timing information detected (step S23)such that the switching timing set up in the mobile station is identicalto the switching timing set up in the base station.

Finally, the spread code switching unit 15c of the transceiver unit 15of the base station carries out the switching of the spread code at theswitching timing set up at the switching timing set up unit 15a, whilethe spread code switching unit 25c of the transceiver unit 25 of themobile station carries out the switching of the spread code at theswitching timing set up at the switching timing set up unit 25b, so asto realize the simultaneous switching of the spread code at the basestation and the mobile station (step S25). After the switching iscompleted, the transceiver unit 15 of the base station and thetransceiver unit 25 of the mobile station notify the completion of thespread code switching to the respective control units 17 and 27.

It is to be noted that, in a case the frame timing is not coinciding inthe reverse link and the forward link, it suffices for the forward linkto carry out the spread code switching on both the base station and themobile station at a prescribed number of frames after the framecontaining the switching timing information that has been changed at thebase station. As for the reverse link, it suffices to carry out thespread code switching on both the base station and the mobile station ata prescribed number of frames after the frame which is closest to theframe in the forward link which contains the switching timinginformation that has been changed at the base station.

Also, in this procedure of FIG. 4, in switching the spread codes at thebase station and the mobile station, the synchronization state beforethe switching is maintained, and the signal transmission is carried outin the same synchronization state even after the switching. In theintra-base station radio link switching, the distance between the basestation and the mobile station is hardly changed immediately before andimmediately after the switching so that the synchronization timing isalso hardly changed. Consequently, the synchronization timing can bemaintained at a time of switching without any problem.

Here, in a case the switching timing information detection processingresult satisfies the out of synchronization condition during the radiolink switching operation, the radio link switching operation is clearedand the synchronization establishment processing in the old radio linkis carried out. However, even when it fails to detect the switchingtiming information in a certain frame during the radio link switchingoperation, as long as the out of synchronization condition is notsatisfied by the time immediately before the spread code switchingtiming, it is judged that it is not out of the synchronization, and theswitching of the spread codes is carried out.

Thus, according to this first embodiment, it is possible to complete theradio link switching within the same transceiver unit 15 of the basestation, so that there is no need for the switching of the wire linkconnected to the transceiver unit 15 at the exchange station side. Also,by carrying out the switching at the same timing on both of the mobilestation and the base station, it is possible to prevent the occurrenceof the period in which the communication is impossible, and further bymaintaining the synchronization state at a time of the switching, it isalso possible to eliminate the synchronization establishment processingfor the new radio link, so that the breaking of the communication due tothe synchronization establishment processing can be prevented. Here, themaintaining the same synchronization state can be done very easily asonly one transceiver unit of the base station is involved in the radiolink switching operation of this first embodiment.

Here, in order to prevent the breaking of the communication, it is alsonecessary for the time required for the radio link switching, i.e., thetime required in changing the radio link parameter within thetransceiver unit, to be sufficiently shorter than the signaltransmission rate. In other words, the upper limit of the signaltransmission rate is determined by the time required in changing theradio link parameter. In this regard, the CDMA is suitable for the highspeed signal transmission rate because the change of the radio linkparameter in the CDMA is the change of the spread code set up in thecorrelator which can be realized at a high speed very easily.Consequently, the use of the CDMA is advantageous in this firstembodiment.

Now, in this first embodiment, the switching timing information to beprovided in each frame used in the above procedure can be given by thelayer 1 bit data in general, and there are several possible choices forthis layer 1 bit data including the following cases.

As a first case, the switching timing information can be given by the socalled unique word (UW) which is provided for the purpose of themaintaining the frame synchronization in the synchronizationestablishment processing, and includes two types of UW0 and UW1. Here,UW0 and UW1 have a sufficient Hamming distance from each other such thatthe probability for confusing these two types is very low.

In this case, as shown in FIG. 6, the transceiver unit 15 of the basestation normally uses UW0 as the switching timing information for theforward link as in the frame 61. Then, at the step S17 of FIG. 4, thetransceiver unit 15 changes the switching timing information to UW1 asin the frame 62. Thereafter, the transceiver unit 15 uses UW0 again asin the subsequent frames 63 to 66.

Similarly, the transceiver unit 25 of the mobile station normally usesUW0 as the switching timing information in the reverse link as in theframe 71.

Meanwhile, the transceiver unit 25 of the mobile station regularlymonitors the switching timing information in the forward link to detectUW0 normally. Then, whenever UW0 cannot be detected, the transceiverunit 25 carries out the detection of UW1, and when UW1 is detected, thetransceiver unit 25 recognizes the start of the radio link switchingoperation. In response, as an acknowledgement of the start of the radiolink switching operation, the transceiver unit 25 changes the switchingtiming information in the forward link to UW1 as in the frame 73. Here,the frame 73 is the first frame in the reverse link for which theswitching timing information can be changed since the detection of UW1in the forward link in the frame 62. Thereafter the transceiver unit 25continues to use UW1 until the spread code switching timing as in theframes 74 and 75.

On the other hand, the transceiver unit 15 of the base station regularlymonitors the switching timing information in the reverse link to detectUW0 normally. However, at the frames 73 to 75 for which UW1 is expectedto be used in the reverse link, the transceiver unit 15 carries out thedetection of UW1, and when UW1 cannot be detected, the transceiver unit15 carries out the detection of UW0. In a case even UW0 cannot bedetected, it is judged that no synchronization according to the uniqueword can be made for that frame so that the synchronization state ismaintained, and it proceeds to carry out the similar operation for thenext frame.

Then, at both of the transceiver unit 15 of the base station and thetransceiver unit 25 of the mobile station, for the forward link, theswitching timing is set as N frames after the frame 62 in which UW1 isused by the base station, whereas for the reverse link, the switchingtiming is set as M frames after the frame 73 in which UW1 can beexpected to be used first by the mobile station.

The transceiver unit 15 of the base station judges that the notificationof the switching timing to the mobile station was successful whenever atleast one UW1 is detected between the frame 73 in which UW1 is expectedto be used first in the reverse link and the frame 75 immediately beforethe switching timing in the reverse link. In this case, the spread codesfor the reverse and forward links are switched to those selected at thestep S5 of FIG. 4 at the respective switching timings as describedabove.

On the contrary, the transceiver unit 15 of the base station judges thatthe notification of the switching timing to the mobile station wasunsuccessful as the transceiver unit 25 of the mobile station failed todetect UW1 in the forward link whenever at least one UW0 is detectedbetween the frame 73 in which UW1 is expected to be used first in thereverse link and the frame 75 immediately before the switching timing inthe reverse link. In this case, the transceiver unit 15 uses UW1 againin the forward link to re-start the radio link switching operation.

On the other hand, the transceiver unit 25 of the mobile stationswitches the spread codes for the reverse and forward links to thosedesignated at the step S7 of FIG. 4 at the respective switching timingsas described above, whenever UW1 is detected in the forward link.

After the switching of the spread codes, the transceiver unit 15 of thebase station and the transceiver unit 25 of the mobile station resumethe communication using UW0 as normal.

It is also possible to modify this first case of using the unique wordas the switching timing information as follows. Namely, as shown in FIG.7, it is possible to use five different types of the unique wordsincluding UW0, UW1, UW2, UW3, and UW4. Here, the different types of theunique word have a sufficient Hamming distance from each other such thatthe probability for confusing any two types is very low.

In this case, as shown in FIG. 7, the transceiver unit 15 of the basestation normally uses UW0 as the switching timing information for theforward link as in the frame 41. Then, at the step S17 of FIG. 4, thetransceiver unit 15 sequentially changes the switching timinginformation to UW4 as in the frame 42, UW3 as in the frame 43, UW2 as inthe frame 44, and UW1 as in the frame 45. Thereafter, the transceiverunit 15 uses UW0 again as in the subsequent frames 46 to 49.

Similarly, the transceiver unit 25 of the mobile station normally usesUW0 as the switching timing information in the reverse link as in theframe 51.

Meanwhile, the transceiver unit 25 of the mobile station regularlymonitors the switching timing information in the forward link to detectUW0 normally. Then, whenever UW0 cannot be detected, the transceiverunit 25 sequentially carries out the detection of UW4 to UW1, and whenany one of UW4 to UW1 is detected, the transceiver unit 25 recognizesthe start of the radio link switching operation. In response, as anacknowledgement of the start of the radio link switching operation, thetransceiver unit 25 sequentially changes the switching timinginformation in the forward link to UW4 as in the frame 53, UW3 as in theframe 54, UW2 as in the frame 55, and UW1 as in the frame 56. Here, theframe 53 is the first frame in the reverse link for which the switchingtiming information can be changed since the detection of UW4 in theforward link in the frame 42. Thereafter the transceiver unit 25continues to use UW1 until the spread code switching timing as in theframes 57 and 58.

Here, the transceiver unit 25 of the mobile station sequentially carriesout the detection of the expected type of the unique word during theperiod since any one of UW4 to UW1 is detected until the spread codeswitching timing (i.e., the period between the frame 42 to the frame48). Thus, in a case UW4 is detected in the frame 42, the detectionprocessing of UW3, UW2, and UW1 is sequentially carried out in theframes 43, 44, and 45, and the detection processing of UW0 is carriedout in the subsequent frames 46 to 48. In a case the expected type ofthe unique word cannot be detected, it is judged that no synchronizationby the unique word can be made in that frame so that the synchronizationstate is maintained, and it proceeds to carry out the similar operationfor the next frame. For instance, when it fail to detect UW3 in theframe 43, the synchronization state is maintained and it proceeds to thedetection processing of UW2 in the frame 44.

On the other hand, the transceiver unit 15 of the base station regularlymonitors the switching timing information in the reverse link to detectUW0 normally. However, at the frames 53 to 58 for which UW4 to UW1 areexpected to be used in the reverse link in correspondence to the UW4 toUW1 used in the forward link, the transceiver unit 15 sequentiallycarries out the detection of UW4 to UW1, Thus, the detection processingof UW4, UW3, and UW2 is sequentially carried out in the frames 53, 54,and 55, and the detection processing of UW1 is carried out in thesubsequent frames 56 to 58. In a case the expected type of the uniqueword cannot be detected in the frame in which UW4 to UW2 are expected tobe used, it is judged that no synchronization by the unique word can bemade in that frame so that the synchronization state is maintained, andit proceeds to carry out the similar operation for the next frame. Forinstance, when it fail to detect UW3 in the frame 54, thesynchronization state is maintained and it proceeds to the detectionprocessing of UW2 in the frame 55. Also, when UW1 cannot be detected inthe frame in which UW1 is expected to be used, the transceiver unit 15carries out the detection of UW0. In a case even UW0 cannot be detected,it is judged that no synchronization by the unique word can be made inthat frame so that the synchronization state is maintained, and itproceeds to carry out the similar operation for the next frame.

Then, at both of the transceiver unit 15 of the base station and thetransceiver unit 25 of the mobile station, for the forward link, theswitching timing is set as N frames after the frame 45 in which UW1 isused by the base station, whereas for the reverse link, the switchingtiming is set as M frames after the frame 56 in which UW1 is expected tobe used first by the mobile station.

The transceiver unit 15 of the base station judges that the notificationof the switching timing to the mobile station was successful whenever atleast one of UW4 to UW1 is detected at the expected frame between theframe 53 in which UW4 is expected to be used in the reverse link and theframe 58 immediately before the switching timing in the reverse link. Inthis case, the spread codes for the reverse and forward links areswitched to those selected at the step S5 of FIG. 4 at the respectiveswitching timings as described above.

On the contrary, the transceiver unit 15 of the base station judges thatthe notification of the switching timing to the mobile station wasunsuccessful whenever at least one UW0 is detected between the frame 53in which UW4 is expected to be used in the reverse link and the frame 58immediately before the switching timing in the reverse link. In thiscase, the transceiver unit 15 uses UW4 again in the forward link tore-start the radio link switching operation.

On the other hand, the transceiver unit 25 of the mobile stationswitches the spread codes for the reverse and forward links to thosedesignated at the step S7 of FIG. 4 at the respective switching timingsas described above, whenever any one of UW4 to UW1 is detected in theforward link.

After the switching of the spread codes, the transceiver unit 15 of thebase station and the transceiver unit 25 of the mobile station resumethe communication using UW0 as normal.

In this modification of the first case, it is possible for thetransceiver unit 25 of the mobile station to recognize the start of thespread code switching operation as well as the spread code switchingtiming by detecting any one of UW4 to UW1 in the forward link in fourframes in which UW4 to UW1 are used, so that even under thecircumstances in which it is difficult to detect the change of theswitching timing information in the forward link due to the degradationof the quality in the radio section, it is possible to increase thechance for successfully completing the radio link switching operation.

Here, in a case the transmission error rate in the radio section ishigh, in order to increase the chance of successfully notifying thestart of the switching operation from the base station to the mobilestation, it is necessary to increase a number of types of the uniqueword used. However, when the number of types of the unique word used isincreased, the switching timing is delayed as much, such that thelowering of the service quality can be caused as the period of time inwhich the quality is degraded becomes longer in the state in which thequality is degraded in the radio section, and the probability for thelink disconnection to occur before the switching is completed becomeshigher. Moreover, as the number of types of the unique word usedincreases, it becomes more difficult to provide an ample Hammingdistance between any two types of the unique word. Consequently, it isnot necessarily preferable to increase the number of types of the uniqueword used unlimitedly, and the suitable number of types of the uniqueword to be used should be determined in view of the above noted factors.

It is also to be noted that, in this first case, the spread codeswitching timing may not necessarily be set to be a predetermined numberof frames after the reference timing at which a particular type of theunique word is used as described above, and can be set to be any desiredtiming if desired.

It is also to be noted that the procedures using the reverse and forwardlinks in this first case as explained with references to FIG. 8 and FIG.7 above are equally applicable to the second, third, and the fourthcases described below.

Next, as a second case, the switching timing information can be given bythe flag in each frame which is in a state of either ON or OFF.

Namely, as shown in FIG. 8, a flag F can be set to be ON in a particularframe, and the simultaneous radio link switching timing for both thebase station and the mobile station is set up to be a prescribed periodof time after this frame with the flag ON. In FIG. 8, the switchingtiming is set to be a timing of a boundary of four frames after thisframe with the flag ON for example.

In this case, the transceiver unit 15 of the base station normally setsthe flag of each frame OFF as the switching timing information, and atthe step S17 of FIG. 4, the transceiver unit 15 changes the switchingtiming information by setting the flag ON. Then, the transceiver unit 15sets the flag of each frame OFF again for the subsequent frames.

On the other hand, the transceiver unit 25 of the mobile stationregularly monitors the switching timing information given by the flag inthe frames transmitted from the base station to check if the flag is ON.Then, whenever the flag is detected to be ON, the transceiver unit 25recognizes the start of the radio link switching operation, and theswitching timing is set to be a prescribed period of time after thisframe with the flag ON, such as a timing of a boundary of four framesafter this frame with the flag ON for example.

Here, instead of constantly fixing the switching timing with respect tothe frame with the flag ON as described above, it is also possible toprovide a multi-valued flag whose value indicates a number of framesuntil the switching timing, as in the flag with a value 5 indicating 5frames to go, the flag with a value 4 indicating 4 frames to go, and soon, for example.

It is also possible to modify this second case of using the flag as theswitching timing information as shown in FIG. 9, in which differentnumbers of flags are used in prescribed number of frames preceding theswitching timing, in order to prevent the overlooking of the switchingtiming information in a form of the flag. For example, in FIG. 9, anumber of flags in each frame indicating a number of frames until theswitching timing, so that a number of flags sequentially decreases asthe switching timing approaches. In this case, even when the first framewith three flags F is overlooked or not recognized due to the poorcommunication state, as long as the subsequent frame with the decreasednumber of flags F can be detected, it is possible to carry out the radiolink switching surely at the prescribed switching timing after the lastframe with only one flag F.

It is also possible to modify this second case of using the flag as theswitching timing information by using the super frame as shown in FIG.10. Namely, in a case of the PDC (Personal Digital Cellular), one superframe is defined by a series of 36 frames, and the flag F can be set. tobe ON in each and every frame of the super frame as indicated in FIG.10, while the switching timing is set to be at a boundary between thissuper frame and a next super frame.

In this second case, the flag F can be located in each frame at aposition as indicated in FIG. 11, between the unique word UW and theassociated control link ACCH provided between the preamble PR and thecommunication link TCH.

Next, as a third case, the switching timing information can be given bythe flag in the so called housekeeping bit which is in a state of eitherON or OFF. The housekeeping bit is an information utilized in manymobile communication system such as the digital mobile telephone systemcurrently in practical use, in order to notify the data such as thereceiving power level or the bit error rate of the communication linkduring the communication between the mobile station and the basestation, for the purpose of controlling the transmission power anddetecting the quality degradation of the radio link at the communicationstation. This housekeeping bit is usually contained in a selected numberof frames among the transmission frames, and formed by using the errorcorrection/detection code so that the highly reliable signaltransmission can be guaranteed for this housekeeping bit. In this thirdcase, the flag is provided in this highly reliable housekeeping bit,such that it becomes possible to realize the highly reliablenotification of the switching timing from the base station to the mobilestation.

In this case, as shown in FIG. 12, the housekeeping bit can be providedin every frame, and the transceiver unit 15 of the base station normallyuses the housekeeping bit R0 with the flag OFF as the switching timinginformation. Then, at the step S17 of FIG. 4, the transceiver unit 15changes the switching timing information by setting the flag in thehousekeeping bit ON (housekeeping bit R1). Then, the transceiver unit 15sets the flag of the housekeeping bit OFF (housekeeping bit R0) againfor the subsequent frames. The simultaneous radio link switching timingfor both the base station and the mobile station is set up to be aprescribed period of time after the housekeeping bit R1 with the flagON. In FIG. 12, the switching timing is set to be a timing of a boundaryof four frames after the frame containing the housekeeping bit R1 withthe flag ON for example.

On the other hand, the transceiver unit 25 of the mobile stationregularly monitors the switching timing information given by the flag inthe housekeeping bit in the frames transmitted from the base station tocheck if the flag is ON. Then, whenever the flag is detected to be ON,the transceiver unit 25 recognizes the start of the radio link switchingoperation, and the switching timing is set to be a prescribed period oftime after this housekeeping bit R1 with the flag ON, such as a timingof a boundary of four frames after the frame containing the housekeepingbit R1 with the flag ON for example.

Here, just as in the second case described above, instead of constantlyfixing the switching timing with respect to the frame containing thehousekeeping bit R1 with the flag ON as described above, it is alsopossible to provide a multi-valued flag whose value indicates a numberof frames until the switching timing, as in the flag with a value 5indicating 5 frames to go, the flag with a value 4 indicating 4 framesto go, and so on, for example.

It is also possible, as shown in FIG. 13, to provide the housekeepingbit periodically in selected frames. In FIG. 13, the transmission signalis given in a unit of a super frame formed by L frames, in which onlythe first frame of the super frame contains the housekeeping bit. Here,as in a case of FIG. 12 described above, the transceiver unit 15 of thebase station normally uses the housekeeping bit R0 with the flag OFF asthe switching timing information, and changes the switching timinginformation by setting the flag in the housekeeping bit ON (housekeepingbit R1) at the first frame of an appropriate super frame. Then, thetransceiver unit 15 sets the flag of the housekeeping bit OFF(housekeeping bit R0) again for the subsequent super frames. Thesimultaneous radio link switching timing for both the base station andthe mobile station is set up to be a prescribed period of time after thehousekeeping bit R1 with the flag ON. In FIG. 13, the switching timingis set to be a timing of a boundary of four frames after the framecontaining the housekeeping bit R1 with the flag ON for example.

Here, just as described above, instead of constantly fixing theswitching timing with respect to the frame containing the housekeepingbit R1 with the flag ON as described above, it is also possible toprovide a multi-valued flag whose value indicates a number of framesuntil the switching timing. For example as shown in FIG. 14, thehousekeeping bit R1-3 having the flag with a value 3 indicates that itis 3 super frames before the switching timing, the housekeeping bit R1-2having the flag with a value 2 indicates that it is 2 super framesbefore the switching timing, and the housekeeping bit R1-1 having theflag with a value 1 indicates that it is 1 super frame before theswitching timing. In this manner, even when the first super frame withthe housekeeping bit R1-3 having the flag with a value 3 is overlookedor not recognized due to the poor communication state, as long as thesubsequent super frame with the housekeeping bit having the flag with adecreased value can be detected, it is possible for the mobile stationto recognize the radio link switching timing, so that the rate ofsuccess of the radio link switching can be increased.

In this third case, the housekeeping bit R is given in a form shown inFIG. 15, Which comprises a receiving power data, the receiving bit errorrate (BER) data, and the flag. Here, the flag can be given by one bit ina case it takes only two states of ON and OFF, or by a plurality of bitsin a case it takes multiple values.

Next, as a fourth case, the switching timing information can be given bythe frame number labelling each frame.

In this case, as shown in FIG. 16, the frames are assigned with serialframe numbers. Then, the transceiver unit 15 of the base stationnormally uses the frame numbers in the serial order as the switchingtiming information. Then, at the step S17 of FIG. 4, as shown in FIG.17, the transceiver unit 15 changes the switching timing information bychanging the frame number of an appropriate frame to be discontinuouswith respect to the frame number of the preceding frame and the framenumber of the following frame. In FIG. 17, the frame number of the framebetween the frame number F7 and the frame number F9 which would normallybe F8 is changed to F50 to make the sequence of the frame numberdiscontinuous. Then, the simultaneous radio link switching timing forboth the base station and the mobile station is set up to be aprescribed period of time after this frame with the changed framenumber. In FIG. 17, the switching timing is set to be a timing of aboundary of four frames after the frame with the changed frame numberF50 for example.

On the other hand, the transceiver unit 25 of the mobile stationregularly monitors the switching timing information given by the framenumber of the frames transmitted from the base station to check if theframe number is discontinuous. Then, whenever the discontinuous framenumber is detected, the transceiver unit 25 recognizes the start of theradio link switching operation, and the switching timing is set to be aprescribed period of time after this frame with the discontinuous framenumber, such as a timing of a boundary of four frames after this framewith the discontinuous frame number for example.

Here, just as in the second and third cases described above, instead ofconstantly fixing the switching timing with respect to the frame withthe changed frame number as described above, it is also possible toassociate meaning related to the number of frames until the switchingtiming to the changed frame number, as in the frame number 50 indicating5 frames to go, the frame number 40 indicating 4 frames to go, and soon, for example. Any other symbols other than the numerals may also beused to associate the meaning to the changed frame number.

It is also possible to change the frame number as shown in FIG. 18, inwhich the serially increasing frame number is changed such that thechanged frame number indicates a number of frames until the switchingtiming in a prescribed number of frames before the switching timing.Thus, in FIG. 18, the frame numbers of three frames after the framenumber F7 which would normally be F8, F9, and F10 in the serial orderare changed to F3, F2, and F1 indicating that it is 3, 2, and 1 framesbefore the switching timing, respectively. In this manner, even when thefirst frame with the changed frame number F3 is overlooked or notrecognized due to the poor communication state, as long as thesubsequent frame with the sequentially decreasing changed frame numbercan be detected, it is possible for the mobile station to recognize theradio link switching timing, so that the rate of success of the radiolink switching can be increased.

It is to be noted here that, in a case of changing the frame number in aplurality of frames before the switching timing, it is necessary to givea due consideration to a number of frames for which the frame numbersare to be changed. Namely, in the normal communication, the frames arepresupposed to be transmitted in an order of the frame numbers, so thatthe change of the frame numbers in only a small number of frames causesno problem but the use of the changed frame numbers in excessively largenumber of frames can make it impossible to carry out the frametransmission and reception order control.

In this fourth case, the frame number FN can be located in each frame ata position as indicated in FIG. 9, between the unique word UW and theassociated control link ACCH provided between the preamble PR and thecommunication link TCH.

It is to be noted here that, in any of the above described first tofourth cases, the switching timing information of the present inventionin general can indicate the time until the switching timing eitherdirectly or indirectly, in one or more frames.

Now, in the first embodiment described above, a manner of determining anumber of frames in which the changed switching timing information is tobe provided during a period between the occurrence of the need for theradio link switching and the switching timing is left unspecified.

In this regard, when the number of frames containing the changedswitching timing information is fixed to a small number, the rate offrame reception failure at the mobile station becomes high in a case theradio link quality is degraded considerably, so that the probability forthe failure of notifying the changed switching timing information to themobile station becomes high and in turn the probability for the failureof the radio link switching operation becomes high.

On the other hand, when the number of frames containing the changedswitching timing information is fixed to a large number, the period oftime required between the radio link switching request and the radiolink switching timing is going to be considerably long all the times, sothat it becomes difficult to carry out the radio link switching atappropriate timings. Moreover, when the frame number is used as theswitching timing information while the number of frames containing thechanged switching timing information is fixed to a large number, theperiod of time during which the designation of the order of frametransmission by the frame number cannot be made becomes considerablylong, and it becomes more likely to cause the trouble in the ordercontrol of the received frames at the mobile station.

In order to remove these potential problems of the first embodiment, thepreferable manner of determining a number of frames in which the changedswitching timing information is to be provided during a period betweenthe occurrence of the need for the radio link switching and theswitching timing can be specified as in the second embodiment of theradio link switching scheme according to the present invention whichwill now be described in detail.

In the following, for the sake of explanation, the second embodimentwill be described for an exemplary case of using the frame number as theswitching timing information as in the fourth case of the firstembodiment described above, the CDMA scheme as the access scheme as inthe first embodiment described above, and the SIR (Signal InterferenceRatio) indicating a ratio of a signal radio wave receiving level and aninterference radio wave receiving level as the quality of the radiolink, although this second embodiment is equally applicable to the othertypes of the switching timing information, the other types of the accessscheme, and the other types of the radio link quality.

In this second embodiment, the radio link switching scheme is carriedout according to the sequence chart of FIG. 20 as follows.

First, the steps S1 to S13 similar to those of FIG. 4 for the firstembodiment described above is carried out.

Then, after the new spread code confirmation is returned from thetransceiver unit 15 to the control unit 17 at the step S13, the controlunit 17 transmits the link quality measurement request signal to thetransceiver unit 15 (step 141). In response the transceiver unit 15measures the SIR of the radio link currently used for communication(step 143), and reports the link quality measurement response signalindicating the measured SIR value to the control unit 17 (step 145).

Next, the control unit 17 determines the switching timing notice framenumber indicating the number of frames for providing the changedswitching timing information, according to the SIR value reported fromthe transceiver unit 15 (step S147). Here, the switching timing noticeframe number is determined according to a prescribed table of the SIRvalue and the switching timing notice frame number as shown in FIG. 21.For example, in a case the measured SIR value is 3.5 dB, the switchingtiming notice frame number is selected to be 3 according to this tableof FIG. 21. In this table of FIG. 21, the switching timing notice framenumber is set to be greater for the lower SIR value and lesser for thehigher SIR value. The control unit 17 then notifies the determinedswitching timing notice frame number to the transceiver unit 15 by theswitching timing notice frame number designation signal (step S149).

Thereafter the steps S15 to S25 similar to those of FIG. 4 for the firstembodiment described above is carried out. In this case, the transceiverunit 15 sets up the switching timing in the switching timing set up unit15a according to the designated switching timing notice frame number atthe step S15A. For example, as shown in FIG. 22, in a case thedesignated switching timing notice frame number is 3, the switchingtiming is set to be as many frames as the designated switching timingnotice frame number after the end of the frame which is transmittedwhile the step S149 is completed. Then, at the step S17, the framenumber of the switching timing notice frames are changed to F3, F2, andF1 indicating the number of frames until the switching timing as in thefourth case of the first embodiment described above.

As described, in this second embodiment, when it is difficult to notifythe radio link switching timing to the mobile station as the radio linkquality such as the SIR value is degraded, the radio link switchingtiming notice frame number is increased, so that the probability for thefailure of notifying the changed switching timing information to themobile station can be reduced and therefore the probability for thefailure of the radio link switching operation can be reduced. On theother hand, when the radio link quality such as the SIR value is notdegraded, the radio link switching timing notice frame number isdecreased, so that the loss of the appropriateness of the radio linkswitching timing due to the unnecessary extension of the switchingtiming can be prevented and therefore the potential for the trouble inthe transmission order control by the frame number at the mobile stationside can be eliminated.

It is to be noted that, instead of the SIR used above, the radio linkquality may be indicated by any parameter from which the frame receptionsuccess probability of the radio link can be estimated such as theabsolute receiving level, the bit error rate (BER), etc.

Also, instead of using the frame number of the fourth case of the firstembodiment described above as the switching timing information, theunique word of the first case, the flag of the second case, or the flagin the housekeeping bit of the third case of the first embodimentdescribed above may be used.

Moreover, instead of indicating the switching timing directly by theframe number as described above, the switching timing may be indicatedindirectly by the frame number according to a prescribed relationshipbetween the switching timing information and the period until theswitching timing.

It is also to be noted that the content of the table of FIG. 21 used indetermining the radio link switching timing notice frame number shouldbe determined according to the required communication performance of themobile communication system, without changing the basic principle ofincreasing the switching timing notice frame number for the lower radiolink quality.

Furthermore, the relationship between the switching timing notice frameand the switching timing may be different from that shown in FIG. 22.For example, as shown in FIG. 23, the switching timing may be set to beas many frames as a predetermined frame number (two in FIG. 23) afterthe end of the last one of the changed switching timing information.This setting of FIG. 23 is useful when the mobile station requires morethan one frame of time for the set up of the switching timing or the newradio link parameter.

In contrast, as shown in FIG. 24, the switching timing may be set to beas many frames as a predetermined frame number (five in FIG. 24) afterthe end of the first one of the changed switching timing informationregardless of the switching timing notice frame number. In this case,the predetermined frame number should be greater than the maximumswitching timing notice frame number that can be set up, for the obviousreason. This setting of FIG. 24 is suitable for a case in which theappropriateness of the radio link switching timing is not so important,but the prevention of the trouble in the transmission order control bythe frame number is important, while it is difficult to control theswitching timing at the mobile station according to the switching timingnotice frame number determined at the base station.

It is also possible to modify the procedure of FIG. 20 described aboveas shown in FIG. 25. Namely, in the procedure of FIG. 25, the radio linkquality for determining the switching timing notice frame number isprovided by the measured quality at a time of the radio link switchingrequest. Thus, in this case, the transceiver unit 15 constantly measuresthe SIR value of the current radio link during the communication withthe mobile station, and the degradation of the radio link quality isdetected at the step S1A by detecting the lowering of the measured SIRvalue below a prescribed threshold. Then, the quality degradation noticesignal including the latest measured SIR value is transmitted from thetransceiver unit 15 to the control unit 17 at the step S3A.

Then, after the spread codes are selected at the step S5, the switchingtiming notice frame number is determined at the step S147 by using thetable of FIG. 21 according to the latest SIR value contained in thequality degradation notice signal. The determined switching timingnotice frame number is notified from the control unit 17 to thetransceiver unit 15 as a part of the new spread code designation signaltransmitted at the step S11A, and the transceiver unit 15 stores thedesignated switching timing notice frame number along with thedesignated spread codes for the reverse and forward links whilereturning the confirmation signal at the step S13A.

Then, at the step S15A, the transceiver unit 15 sets up the switchingtiming in the switching timing set up unit 15a according to thedesignated switching timing notice frame number Just as in a case ofFIG. 20 described above, and the steps S17 to S25 similar to those ofFIG. 4 for the first embodiment described above are carried outthereafter.

It is also to be noted here that, besides those already mentioned above,many modifications and variations of the above embodiments may be madewithout departing from the novel and advantageous features of thepresent invention. Accordingly, all such modifications and variationsare intended to be included within the scope of the appended claims.

We claim:
 1. A method for switching a radio link in a mobilecommunication between a base station and a mobile station, comprisingthe steps of:carrying out a communication between the base station andthe mobile station through the radio link given in terms of frames whileproviding a switching timing information formed by a layer 1 bit data ineach frame of the radio link; changing the switching timing informationin at least one of the frames of the radio link at the base station toindicate a switching of the radio link whenever a need for switching theradio link between the base station and the mobile station arises;transmitting the changed switching timing information from the basestation to the mobile station; switching the radio link at the basestation to a new radio link at a switching timing which is a firstprescribed period of time after each frame in which the switching timinginformation is changed at the changing step; detecting the changedswitching timing information transmitted from the base station at themobile station; and switching the radio link at the mobile station tothe new radio link at a switching timing which is a second prescribedperiod of time after each frame in which the changed switching timinginformation is detected at the detecting step.
 2. The method of claim 1,wherein the switching timing information is given by a unique word formaintaining a synchronization of the frames.
 3. The method of claim 2,wherein the unique word is normally set to be one type and changed toanother type different from said one type at the changing step.
 4. Themethod of claim 2, wherein the unique word is normally set to be onetype and the unique word in each of a plurality of consecutive framesare sequentially changed to other types different from said one type atthe changing step.
 5. The method of claim 1, further comprising thesteps of:changing the switching timing information in at least one ofthe frames of the radio link at the mobile station in response to adetection of the changed switching timing information at the detectingstep; and transmitting the changed switching timing information from themobile station to the base station; wherein the base station carries outthe switching of the radio link only when the changed switching timinginformation from the mobile station is received before the switchingtiming, and otherwise repeats the step of transmitting the changedswitching timing information from the base station to the mobilestation.
 6. The method of claim 1, wherein the switching timinginformation is changed at the changing step such that a value of thechanged switching timing information indicates a time until theswitching timing.
 7. The method of claim 6, wherein the switching timinginformation is changed at the changing step in a plurality of frames. 8.The method of claim 1, wherein the switching timing information is givenby a flag provided in each frame.
 9. The method of claim 8, wherein theflag is provided in a housekeeping bit of each frame for notifying areception state of the radio link between the base station and themobile station.
 10. The method of claim 8, wherein the flag is normallyset to take one value and changed to take another value different fromsaid one value at the changing step.
 11. The method of claim 8, whereinthe flag takes any one of multiple values, and is changed at thechanging step such that a value of the changed flag in each frameindicates a number of frames from said each frame to the switchingtiming.
 12. The method of claim 11, wherein the flag is changed at thechanging step in a plurality of consecutive frames with sequentiallydecreasing value of the flag.
 13. The method of claim 1, wherein theswitching timing information is given by a plurality of flags providedin each frame, which are changed at the changing step in a plurality ofconsecutive frames with sequentially decreasing number of the changedflags in the consecutive frames.
 14. The method of claim 1, wherein theswitching timing information is changed at the changing step in units ofsuper frames formed by the frames.
 15. The method of claim 1, whereinthe switching timing information is given by a frame number assigned toeach frame.
 16. The method of claim 15, wherein the frame number isnormally assigned sequentially in an order of transmission of theframes, and changed at the changing step to take a value which is notsequential with respect to the frame numbers of immediately precedingand following frames.
 17. The method of claim 15, wherein the framenumber is changed at the changing step such that a value of the changedframe number in each frame indicates a number of frames from said eachframe to the switching timing.
 18. The method of claim 17, wherein theframe number is changed at the changing step in a plurality ofconsecutive frames with sequentially decreasing value of the framenumber.
 19. The method of claim 1, wherein the base station and themobile station maintain a synchronization state before and after theswitching of the radio link to be identical.
 20. The method of claim 1,wherein the switching timing information is changed at the changing stepin a variable number of frames.
 21. The method of claim 20, furthercomprising the steps of:measuring a quality of the radio link; andcontrolling the variable number of frames in which the switching timinginformation is to be changed at the changing step according to thequality measured at the measuring step.
 22. The method of claim 21,wherein at the controlling step, the variable number is increased for alower value of the quality measured at the measuring step, and decreasedfor a higher value of the quality measured at the measuring step. 23.The method of claim 21, wherein at the controlling step, the variablenumber is controlled by using a table specifying a desired number offrames in which the switching timing information is to be changed withrespect to each level of the quality measured by the measuring step. 24.The method of claim 21, wherein the measuring step measures the qualityat a timing at which the need for switching the radio link between thebase station and the mobile station arises.
 25. The method of claim 21,wherein the measuring step measures the quality at a timing before theswitching timing information is changed at the base station.
 26. Themethod of claim 1, wherein the switching timing is specified in advanceas a prescribed number of frames from a start of each frame containingthe changed switching timing information.
 27. The method of claim 1,wherein the switching timing is specified in advance as a prescribednumber of frames from an end of each frame containing the changedswitching timing information.
 28. A mobile communication systemincluding a base station and a mobile station, comprising:means forcarrying out a communication between the base station and the mobilestation through the radio link given in terms of frames while providinga switching timing information formed by a layer 1 bit data in eachframe of the radio link; means for changing the switching timinginformation in at least one of the frames of the radio link at the basestation to indicate a switching of the radio link whenever a need forswitching the radio link between the base station and the mobile stationarises; means for transmitting the changed switching timing informationfrom the base station to the mobile station; means for switching theradio link at the base station to a new radio link at a switching timingwhich is a first prescribed period of time after each frame in which theswitching timing information is changed by the changing means; means fordetecting the changed switching timing information transmitted from thebase station at the mobile station; and means for switching the radiolink at the mobile station to the new radio link at a switching timingwhich is a second prescribed period of time after each frame in whichthe changed switching timing information is detected by the detectingmeans.
 29. The system of claim 28, wherein the switching timinginformation is given by a unique word for maintaining a synchronizationof the frames.
 30. The system of claim 29, wherein the unique word isnormally set to be one type and the changing means changes the uniqueword to another type different from said one type.
 31. The system ofclaim 29, wherein the unique word is normally set to be one type and thechanging means changes the unique word in each of a plurality ofconsecutive frames sequentially to other types different from said onetype.
 32. The system of claim 28, further comprising:means for changingthe switching timing information in at least one of the frames of theradio link at the mobile station in response to a detection of thechanged switching timing information by the detecting means; and meansfor transmitting the changed switching timing information from themobile station to the base station; wherein at the base station, theswitching of the radio link is carried out only when the changedswitching timing information from the mobile station is received beforethe switching timing, and otherwise a transmission of the changedswitching timing information from the base station to the mobile stationis repeated.
 33. The system of claim 28, wherein the changing meanschanges the switching timing information such that a value of thechanged switching timing information indicates a time until theswitching timing.
 34. The system of claim 33, wherein the changing meanschanges the switching timing information in a plurality of frames. 35.The system of claim 28, wherein the switching timing information isgiven by a flag provided in each frame.
 36. The system of claim 35,wherein the flag is provided in a housekeeping bit of each frame fornotifying a reception state of the radio link between the base stationand the mobile station.
 37. The system of claim 35, wherein the flag isnormally set to take one value and the changing means changes the flagto take another value different from said one value.
 38. The system ofclaim 35, wherein the flag takes any one of multiple values, and thechanging means changes the flag such that a value of the changed flag ineach frame indicates a number of frames from said each frame to theswitching timing.
 39. The system of claim 38, wherein the changing meanschanges the flag in a plurality of consecutive frames with sequentiallydecreasing value of the flag.
 40. The system of claim 28, wherein theswitching timing information is given by a plurality of flags providedin each frame, and the changing means changes the flags in a pluralityof consecutive frames with sequentially decreasing number of the changedflags in the consecutive frames.
 41. The system of claim 28, wherein thechanging means changes the switching timing information in units ofsuper frames formed by the frames.
 42. The system of claim 28, whereinthe switching timing information is given by a frame number assigned toeach frame.
 43. The system of claim 42, wherein the frame number isnormally assigned sequentially in an order of transmission of theframes, and the changing means changes the frame number to take a valuewhich is not sequential with respect to the frame numbers of immediatelypreceding and following frames.
 44. The system of claim 42, wherein thechanging means changes the frame number such that a value of the changedframe number in each frame indicates a number of frames from said eachframe to the switching timing.
 45. The system of claim 44, wherein thechanging means changes the frame number in a plurality of consecutiveframes with sequentially decreasing value of the frame number.
 46. Thesystem of claim 28, wherein the base station and the mobile stationmaintain a synchronization state before and after the switching of theradio link to be identical.
 47. The system of claim 28, wherein thechanging means changes the switching timing information in a variablenumber of frames.
 48. The system of claim 47, further comprising:meansfor measuring a quality of the radio link; and means for controlling thevariable number of frames in which the switching timing information isto be changed by the changing means according to the quality measured bythe measuring means.
 49. The system of claim 48, wherein the controllingmeans increases the variable number for a lower value of the qualitymeasured by the measuring means, and decreases the variable number for ahigher value of the quality measured by the measuring means.
 50. Thesystem of claim 48, wherein the controlling means controls the variablenumber by using a table specifying a desired number of frames in whichthe switching timing information is to be changed with respect to eachlevel of the quality measured by the measuring means.
 51. The system ofclaim 48, wherein the measuring means measures the quality at a timingat which the need for switching the radio link between the base stationand the mobile station arises.
 52. The system of claim 48, wherein themeasuring means measures the quality at a timing before the switchingtiming information is changed at the base station.
 53. The system ofclaim 28, wherein the switching timing is specified in advance as aprescribed number of frames from a start of each frame containing thechanged switching timing information.
 54. The system of claim 28,wherein the switching timing is specified in advance as a prescribednumber of frames from an end of each frame containing the changedswitching timing information.